U.S. patent number 4,274,419 [Application Number 06/086,590] was granted by the patent office on 1981-06-23 for skin preparation device and method used in the application of medical electrodes.
This patent grant is currently assigned to Quinton Instrument Co.. Invention is credited to Vernon E. Modes, Hak W. Tam.
United States Patent |
4,274,419 |
Tam , et al. |
June 23, 1981 |
Skin preparation device and method used in the application of
medical electrodes
Abstract
A method and device for preparing the skin of a patient to
ensure good electrical contact between a medical electrode and the
patient's skin. The electrode or the tips of the preparation device
includes an abrasive pad impregnated with an electrolyte for
providing good electrical contact with the patient's skin. When an
electrode is used, the pad is centrally mounted and is rotated in a
housing having an adhesive annular rim which is secured to the
skin. The pad is rotatably driven by the preparation device which
continuously measures the impedance between the pad of the
electrode and a reference electrode, which is also secured to the
skin. A motor in the preparation device rotates the pad to abrade
the skin responsive to actuation of a switch. Rotation of the pad
terminates within a predetermined period or when the impedance
between the pad and reference electrode falls to a predetermined
value, whichever occurs first. The impedance between the electrode
being applied and the reference electrode is measured by producing
an alternating current having a constant magnitude between the
electrodes, rectifying the voltage across the electrodes, which is
proportional to the impedance therebetween and the voltage, to an
adjustable reference voltage. When the rectified voltage drops to
the reference voltage a manually actuated timer powering the motor
is reset if the timer has not already been reset after the
predetermined period.
Inventors: |
Tam; Hak W. (Kirkland, WA),
Modes; Vernon E. (Kent, WA) |
Assignee: |
Quinton Instrument Co.
(Seattle, WA)
|
Family
ID: |
22199579 |
Appl.
No.: |
06/086,590 |
Filed: |
October 19, 1979 |
Current U.S.
Class: |
600/372;
600/547 |
Current CPC
Class: |
A61B
5/25 (20210101); A61B 5/053 (20130101); A61B
5/276 (20210101); A61B 5/325 (20210101) |
Current International
Class: |
A61B
5/0402 (20060101); A61B 5/0424 (20060101); A61B
5/0408 (20060101); A61B 005/04 () |
Field of
Search: |
;128/639-641,644,783,798,802,803,734,33R,303.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Tam et al., "Minimizing Electrode Motion Artifact . . . ", IEEE
Trans. on Bio. Med. Eng., vol. 24, No. 2, Mar. 1977, 134-139. .
Burbank et al., "Reducing Skin Potential Motion Artifact . . . ",
Med. and Biol. Eng. and Comput., 1978, 16, 31-38..
|
Primary Examiner: Cohen; Lee S.
Attorney, Agent or Firm: Seed, Berry, Vernon &
Baynham
Claims
We claim:
1. A method of preparing the skin of a patient to provide proper
electrical contact between the skin and a medical electrode to be
secured to the skin of the patient, comprising:
applying a reference electrode to a prepared site on the skin of
the patient;
abrading an unprepared site on the skin of the patient with an
abrasive pad impregnated with an electrolyte;
measuring the difference of the impedance between the abrasive pad
and the reference electrode during abrading of the skin with the
abrasive pad; and
terminating abrasion with the abrasive pad when the difference of
the impedance between the abrasive pad and the reference electrode
reaches a preset value.
2. A method of preparing the skin of a patient to provide proper
electrical contact between the skin and a monitoring medical
electrode, comprising:
applying a reference electrode to a previously prepared site on the
skin of the patient, the reference electrode having means
establishing an electrical connection between the prepared site on
the skin and the reference electrode for sensing and conveying
bioelectric signals;
applying a monitoring electrode to an unprepared site on the skin
of the patient, the monitoring electrode having electrically
conductive means for sensing and conveying bioelectric signals, the
electrically conductive means including abrading means in contact
with the skin for abrading the portion of the skin in contact
therewith;
abrading the portion of the skin contacted by the abrading means of
the monitoring electrode;
measuring the relative difference in impedance at the abrading
means/skin interface relative to the impedance at the reference
electrode/skin interface during abrading of the skin with the
abrading means; and
terminating abrasion of the skin with the abrading means when the
difference in impedance between the abrading means/skin interface
and reference electrode/skin interface falls to a preset value.
3. A method of preparing the skin of a patient to provide proper
electrical contact between the skin and a monitoring medical
electrode, the monitoring electrode having electrically conductive
means for sensing and conveying bioelectric signals, including an
abrasive pad impregnated with an electrolyte mounted for rotation
in contact with the skin of the patient after application of the
monitoring electrode to the skin, comprising:
applying a reference electrode to a previously prepared site on the
skin of the patient, the reference electrode having electrically
conductive means in contact with the prepared site on the skin of
the patient for sensing and conveying bioelectric signals;
applying a monitoring electrode to an unprepared site on the skin
of the patient with the electrolyte-impregnated abrasive pad in
contact with the unprepared skin of the patient;
rotating the abrasive pad of the monitoring electrode;
continuously measuring the relative difference of the impedance at
the abrasive pad/skin interface of the monitoring electrode
relative to the impedance at the electrically conductive means/skin
interface of the reference electrode during rotation of the
abrasive pad; and
terminating rotation of the abrasive pad when the relative
difference in the impedance between the abrasive pad/skin interface
and electrically conductive means/skin interface falls to a preset
value.
4. The method of claim 3, wherein the AC impedance between the
monitoring electrode and reference electrode is measured such that
DC potentials generated by the monitoring electrode and reference
electrode do not affect the impedance measurements.
5. The method of claim 4, wherein the impedance measurements are
accomplished by the steps of generating an alternating current
having a constant magnitude between the monitoring electrode and
reference electrode and measuring the voltage across the monitoring
electrode and reference electrode.
6. The method of claim 5, wherein the alternating current is of a
magnitude and frequency that avoids physiological changes or
responses of the skin tissue.
7. The method of claim 3, further including the step of terminating
rotation of the abrasive pad of the monitoring electrode after a
predetermined period regardless of whether the impedance between
the monitoring electrode and reference electrode has fallen to the
preset value in order to prevent excessive abrasion of the skin in
the event of an improper impedance measurement.
8. An apparatus for preparing the skin of a patient to provide
proper electrical contact between the skin and an abrasive pad
impregnated with an electrolyte in contact with and movable
relative to the skin of the patient, comprising:
an electrically conductive drive member for moving the abrasive pad
relative to the skin of the patient to abrade the portion of the
skin in contact with the abrasive pad;
means for completing an electrical circuit between the abrasive pad
and the patient through the electrically conductive drive member,
the means including a reference electrode having means establishing
an electrical connection between a prepared site on the skin of the
patient and the reference electrode; and
impedance measuring means for measuring the relative difference of
the impedance at the abrasive pad/skin interface and the electrical
connection means/skin interface of the reference electrode, the
electrically conductive drive member moving the abrasive pad to
reduce the impedance at the abrasive pad/skin interface until such
impedance falls to a preset reference value relative to the
impedance at the electrical connection means/skin interface of the
reference electrode.
9. An apparatus for preparing the skin of a patient to provide
electrical contact between the skin and a monitoring medical
electrode secured to the skin, the electrode including an abrasive
pad impregnated with an electrolyte in contact with and movable
relative to the skin of the patient for sensing and conveying
bioelectric signals, comprising:
an electrically conductive drive member adapted to be electrically
connected to the electrolyte-impregnated abrasive pad of the
monitoring electrode to move the abrasive pad of the electrode
relative to the skin of the patient;
an electric motor having an output shaft connected to the drive
member for rotating the abrasive pad;
impedance measuring means for measuring the impedance between the
drive member and a reference electrode having conductive means
establishing an electrical connection between a prepared site on
the skin of the patient and the reference electrode, the reference
electrode electrically connected to the drive member so that the
measurement of the relative impedance between the drive member and
reference electrode provides an indication of the impedance at the
abrasive pad/skin interface of the monitoring electrode;
initiating means for causing the motor to move the drive member
when electrically connected to the monitoring electrode;
comparator means for generating a terminating signal when the
impedance, as measured by the impedance measuring means, falls to
the preset reference value; and
control means for preventing the motor from continuing to move the
drive member responsive to the terminating signal.
10. The apparatus of claim 9, wherein the drive member includes
means for maintaining the pressure of the pad against the skin of
said patient relatively constant.
11. The apparatus of claim 9, wherein the shaft of the motor
functions as an electrically conductive path.
12. The apparatus of claim 9, wherein the impedance measuring means
includes a constant current source connected in series with the
drive member and electrically connectable to the reference
electrode and voltage measuring means for measuring the voltage
between the monitoring electrode electrically connectable to drive
member and the reference electrode.
13. The apparatus of claim 9, including an electric plug to which
the reference electrode is connectable to the impedance measuring
means, the electrical plug including means for mating with an
electric jack, the jack having a first terminal connected to a
second terminal when the plug is not inserted in the jack, and
which is connected to the plug and electrically isolated from the
second terminal when the plug is inserted in the jack, the second
terminal being connected to the drive member through a relatively
low impedance path so that the motor is prevented from moving the
pad of the monitoring electrode when the reference electrode is not
connected to the impedance measuring means.
14. The apparatus of claim 9, wherein the impedance mesuring means
includes an oscillator for generating an AC signal between the
monitoring electrode connectable to the drive member and the
reference electrode through an impedance sufficiently large so that
the current flowing between the monitoring and reference electrode
is insensitive to variations in the impedance at the interface
between the abrasive pad of the monitoring electrode and the skin
of the patient, and the voltage between the drive member and the
reference electrode is indicative of the impedance at the interface
between the skin/abrasive pad interface.
15. The apparatus of claim 9, wherein the impedance measuring
means, comparator means and control means comprise:
a circuit element having an impedance which is substantially
greater than the maximum impedance between the drive member and the
reference electrode;
an oscillator for generating an AC signal between the monitoring
electrode connectable to the drive member and the reference
electrode through the circuit element;
a rectifier receiving a voltage from the reference electrode for
producing an DC voltage which is proportional to the impedance
between the reference electrode and the monitoring electrode
connectable to the drive member;
a comparator receiving the output of the rectifier, the comparator
generating a terminate signal when the DC voltage from the
rectifier falls to a preset level;
manually controllable means for generating an initiate signal;
timer means for generating an actuate signal responsive to the
initiate signal, the actuate signal continuing for a predetermined
period or until the termination signal is generated by the
comparator, whichever occurs first; and
power circuit means operatively connected to the motor for applying
power to the motor responsive to the initiate signal.
16. The apparatus of claim 15, further including a high pass filter
positioned between the reference electrode and the rectifier for
preventing DC voltages generated by the monitoring electrode and
the reference electrode from interfering with the impedance
measurements.
17. The apparatus of claim 9, including a housing containing the
electrically conductive drive member, electric motor, measuring
means, initiating means, comparator means, and control means, the
housing having a piston grip configuration for ease of use.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the preparation of a patient's skin to
ensure good electrical contact with a medical electrode, and more
particularly to a device for automatically abrading the skin until
the impedance at the interface between the skin and the abrading
means drops to a predetermined value.
2. Description of the Prior Art
Medical electrodes are commonly used to perform a large number of
medical tests, such as electrocardiograms. These electrodes are
generally of the disposable variety having an
electrolyte-impregnated pad surrounded by a housing which is
adhesively secured to the skin. In order for the electrode to
properly sense subcutaneous electrical signals, the skin of the
patient must be prepared by abrading the skin on which the
electrode is to be placed. This abrasion removes foreign matter and
a layer of dead skin to ensure better electrical contact between
the electrolyte pad and the patient's skin.
Skin preparation for medical electrodes is presently accomplished
by manually rubbing the patient's skin with a pad containing a
cleansing agent. The primary disadvantages of this technique are
the length of time required to prepare the patient for a medical
test utilizing medical electrodes and the inability to precisely
control the degree of skin preparation. In many medical tests, such
as an electrocardiogram, a large number of electrodes--on the order
of 3 to 14--must be secured to the patient's skin. Preparation of
the patient's skin by manual means requires a great deal of time,
thereby making such tests fairly costly. Manual preparation of the
skin is continued until it appears to the person applying the
electrodes that the skin has been abraded sufficiently. However,
since this visual technique is rather imprecise, skin abrasion is
either insufficient to allow the electrode to accurately receive
voltage levels or excessive, causing patient discomfort.
A new type medical electrode is described in copending application,
Ser. No. 003,109, filed Jan. 15, 1979, and entitled "MEDICAL
ELECTRODE AND SYSTEM FOR MINIMIZING MOTION ARTIFACTS". This medical
electrode includes an abrasive pad impregnated with an electrolyte
solution which is rotatably mounted in a cylindrical housing. The
housing is surrounded by an annular rim having an adhesive coating
securing the electrodes to the skin. Skin preparation is easily and
quickly accomplished by rotating the pad against the skin of the
patient after the electrode has been secured to the skin. One
problem associated with use of the above-described electrode is the
difficulty in determining when the skin has been sufficiently
prepared. The pad of the electrode may be rotated manually to
abrade the skin or preferably be rotated by power means such as an
electric motor. Individual patients require different amounts of
preparation because of their differing skin characteristics; thus,
a constant amount of skin preparation by the abrasive pad of the
electrode may be excessive in some cases and insufficient in
others.
SUMMARY OF THE INVENTION
The basic object of this invention is to quickly, easily and safely
apply a large number of medical electrodes of the type described to
the skin of a patient in a manner which ensures adequate and
uniform skin preparation for each applied electrode.
It is another object of the invention to provide a skin preparation
device for abrading the skin of a patient through an abrasive
gel-impregnated pad only until a specific degree of skin
preparation has been accomplished.
It is still another object of the invention to provide a method and
device for continuously measuring the degree of skin preparation
during the skin preparation procedure prior to or during
application of medical monitoring electrodes.
These and other objects of the invention are provided by
continuously measuring the impedance of the interface between an
abrasive pad attached to a preparation device or the sensing means
of a monitoring electrode and the skin of the patient during skin
preparation relative to an applied reference electrode. This is
accomplished by first applying a reference electrode to the skin
after adequate skin preparation, either by manual means or
mechanized preparation where the electrode has a rotatably mounted
pad as described in the above-mentioned co-pending application.
Secondly, the skin where the electrodes are to be applied is
abraded with an abrasive pad secured to the tip of the preparation
device or impregnated with electrolyte and a large number of
monitoring electrodes are applied to the skin of the patient with
the sensing element of each of the monitoring electrodes moved
relative to the skin in contact with the sensing element by the
skin preparation device while the impedance between the abrading
tip or sensing element and the reference electrode is continuously
measured. When the impedance falls to a predetermined value,
further abrasion of the skin by the device is terminated. The
impedance measurements are accomplished by generating an
alternating current having a constant magnitude between the
reference electrode and the monitoring electrode being applied and
measuring the voltage across the electrodes, the voltage being
proportional to the impedance. The voltage is then rectified and
compared with a manually adjustable reference voltage. When the
impedance voltage at the interface between the monitoring electrode
and skin falls to the level of that of the reference voltage, power
is removed from the electric motor of the skin preparation device
used to abrade the skin. The skin preparation device also includes
a circuit for creating a low impedance relative to the impedance at
the interface between the monitoring electrode and skin if the
reference leadwire is not connected to the skin preparation device.
A high pass filter is preferably positioned between the reference
electrode and the rectifier for preventing DC voltages generated by
the reference electrode and monitoring electrode being applied from
interfering with the impedence measurements. As an additional
safety feature to prevent excessive skin preparation, rotation of
the motor of the skin preparation device may be terminated after a
predetermined time period if the impedance at the interface between
the monitoring electrode and skin has not fallen to the preset
value.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an isometric view of the skin preparation device as used
on a patient in preparing the skin of the patient by abrading the
skin of the patient beneath the sensing element of the monitoring
electrode.
FIG. 2 is a side elevational view of the skin preparation device
for use with a medical electrode capable of preparing the skin of a
patient after application thereof.
FIG. 3 is a schematic of the circuit used in the skin preparation
device of FIG. 2.
DETAILED DESCRIPTION OF THE INVENTION
As illustrated in FIG. 1, a medical monitoring electrode 10 of the
type having means to abrade the skin of the patient is secured to a
patient P, and the skin beneath the sensing element of the
electrode 10 is prepared by a medical technician utilizing the skin
preparation device 12. The monitoring electrode may be of the type
disclosed in patent application Ser. No. 003,109 (previously
mentioned), the disclosure of which is hereby incorporated by
reference, or an electrode of other design having means to prepare
the skin of the patient contacted by the sensing element of the
electrode after the electrode is secured to the patient. A leadwire
14 extends from the device 12 to a reference electrode 16 which has
been previously secured to the patient P after the patient's skin
has been prepared. If the reference electrode 16 is of the
conventional type, the skin beneath the electrode 16 is prepared
utilizing the conventional manual technique. If the reference
electrode 16 is of the rotatable abrasive pad variety described in
patent application Ser. No. 003,109, the skin is prepared by
abrading the skin with the rotating pad either manually or by power
means for a predetermined period. The device 12 rotates the
rotatably mounted pad of the electrode 10 while continuously
measuring the impedance between the electrode 10 and the reference
electrode 16. When the electrode 10 is initially applied to the
patient's skin, the impedance is relatively high, primarily due to
a surface layer of dead skin contacting the pad of the electrode
10. As this layer of dead skin is abraded by rotation of the pad,
the impedance is reduced. The device 12 includes means for
determining when the impedance is lowered to a preset level, at
which time rotation of the pad of the electrode 10 is
terminated.
The skin preparation device for rotating the rotatably mounted
abrasive pad of the medical electrode is illustrated in greater
detail in FIG. 2. The device 12 includes a conventional DC motor 20
with electrically conductive housing and shaft which may be, for
example a 3-Amp, 4.8 volt motor manufactured by the Barber-Colman
Company of Rodeford, Illinois, which spins at about 3,800 RPM at 18
inch-ounce load condition. The motor 20 drives a conventional
reduction gear assembly 22 which may have a reduction ratio, for
example, of about 8.33:1. The reduction gear assembly 22 includes
an output shaft 24 on which an electrically conductive drive
assembly 26 is mounted. The drive assembly 26 is connected to one
terminal of the impedance sensing circuit so that skin impedance
can be measured through electrode 10 as it rotates the pad of the
electrode. The drive assembly 26 includes a drive member 28 having
a drive head 28a and an integral cylindrical portion 28b of smaller
diameter than the drive head 28a slidably mounted on the output
shaft 24. An annular collar 30 of larger diameter than the
cylindrical portion 28b surrounds the cylindrical portion 28b and
is retained on the output shaft 24 by a spoke 32 which extends
through a slot 34 formed in the cylindrical portion 28b of the
drive member 28. The drive member 28 is freely slidable on the
output shaft 24 in an axial direction. A compression spring 36
extends between the drive head 28a and the collar 30 to resiliently
bias the drive head 28a in an axial direction away from the collar
30. This resilient biasing allows the drive head 28a to exert a
predetermined, relatively constant pressure on the abrasive pad of
the electrode being applied to the patient as it rotates the pad.
The drive assembly 26 also furnishes a electrically conductive path
for continuous impedance sensing purpose.
Power is initially applied to the motor 20 by a battery pack 40
when a conventional switch 42 is closed by depressing a trigger 44
carried by an actuating shaft 46 projecting from the switch 42. The
battery pack 40 is preferably composed of rechargeable batteries
which are charged by a conventional battery charger connected to
the battery pack 40 through a conventional charging jack 48. A
second jack 50 is provided to receive the leadwire 14 (FIG. 1)
extending from the reference electrode 16 and functioning as a
leadwire sentry circuit. The jack 50 is preferably of the type
commonly used with small radios and tape recorders for receiving an
earphone plug. Jacks of this type automatically disconnect the
audio output from a loudspeaker and instead apply it to the
earphone. As explained in greater detail hereinafter, when a plug
is not inserted in the jack 50 the impedance measuring circuitry
receives a signal indicative of an extremely low impedance so that
the motor 20 does not rotate. When a plug is inserted into the jack
50, the impedance measuring circuitry receives a signal indicative
of the actual impedance between the electrode 10 and the reference
electrode 16. Operation of the skin preparation device is thus
prevented unless the reference electrode plug has been inserted in
the jack 50. The above described leadwire sentry circuit reduces
the risk of operator error as well as conserving battery power when
the device is inadvertantly triggered without the reference wire
attached.
The circuitry for measuring impedance and controlling the operation
of the motor 20 is mounted on a printed-circuit board 52 with the
circuit components, indicated generally at 54, pointing downwardly
as illustrated in FIG. 2.
All of the above-described components of the skin preparation
device 12 are mounted in a housing having a pistol grip handle
portion 56a, a motor and circuit enclosure 56b and a drive assembly
enclosure 56c which is preferably fabricated from a transparent
material. The housing is preferably formed using a durable,
insulating material.
The circuitry for measuring impedance and controlling the operation
of the motor 20, generally indicated at 54 in FIG. 2, is shown
schematically in FIG. 3. An AC signal is generated by a
conventional oscillator 60 and applied to the jack 50 through a
relatively high impedance resistor 62 and a DC blocking capacitor
64. The frequency of the oscillation is preferably about 55 Hz to
minimize interference from extraneous AC power signals which are
either 50 to 60 Hz for most countries. The resistor 62 preferably
has a resistance which is substantially greater than the maximum
impedance through the body of the patient between the reference
electrode 16 and the applied electrode 10. When the electrode 10 is
initially applied to the patient's skin, the impedance between the
electrode 16 and electrode 10 (it being remembered that the
conductive drive head 28a (FIG. 1) electrically conducts between
the electrode 10 and one terminal of this impedance sensing circuit
whereas the reference wire 14, attached to electrode 16,
electrically conducts between electrode 16 and the other terminal
of the impedance sensing circuit) is generally between 50,000 ohms
and 200,000 ohms. When the skin of the patient beneath the
electrode 10 has been properly prepared, however, the impedance
between the electrodes 10 and 16 drops to about 3,500 ohms. By
making the impedance of resistor 62 substantially larger than
200,000 ohms--on the order of 1 megaohm--the current through the
patient is relatively insensitive to variations in the
interelectrode impedance. The oscillator 60 and resistor 62 thus
function as a non-polarizating constant.
The medical electrode 10, as with other types of electrodes,
generates a DC potential which may interfere with proper impedence
measurements. These DC potentials are elimnated by the blocking
capacitor 64.
Since the current between the electrodes 16 and 10 is constant, the
voltage at the reference electrode 16 is directly proportional to
the impedance between the electrodes and this impedance is
indicative of the impedance at the interface between the electrode
10 and the skin of the patient. This voltage is applied to a
conventional voltage amplifier 66 through a wide band pass filter
formed by capacitors 68 and 70. Capacitor 68 attenuates noise
signals having frequency components far in excess of 55 Hz, while
capacitor 70 attenuates relatively low frequency signals and DC
potentials generated by the electrodes 16 and 10. The output of
amplifier 66 is half-wave rectified by a diode 72 to produce a DC
voltage which is indicative of the impedance of the skin/electrode
10 interface. This DC voltage is applied to one terminal of a
conventional voltage comparator 74 which is low pass filtered by
capacitor 76 and resistor 78. Capacitor 76 causes the input to the
comparator 74 to be relatively constant even though current flows
through the diode 72 for only a small portion of each AC cycle.
Resistor 78 provides a discharge path to ground so as to slowly
discharge the capacitor 76. The other input to comparator 74 is a
DC reference voltage from the wiper of a potentiometer 80 which is
connected between a supply voltage and ground to form a voltage
divider. The wiper of the potentiometer 80 is manually adjustable
to select a predetermined impedance point at which the output of
the comparator 74 switches. The output of the comparator 74 is
applied to the reset terminal of a conventional one-shot or timer
82. The trigger input to the one-shot 82 is selectively grounded by
the switch 42 of FIG. 2. When the switch 42 is actuated, the
one-shot 82 drives a current amplifier 84 causing current to flow
through the motor 20 to rotate the rotatably mounted pad of the
electrode 10. After a predetermined period, as set by the internal
components of the one-shot 82, the output of the one-shot 82 falls
to 0 volts, causing the current amplifier 84 to cut off power to
the motor 20. If the impedance between the electrodes 16 and 10
falls to the reference level set by the potentiometer 80 before the
end of this predetermined period, the one-shot 82 is reset to cause
the current amplifier 84 to cut off power to the motor 20.
The current amplifier 84 includes a first transistor 86 which is
normally biased "off" through resistor 88. When the one-shot 82 is
triggered, a positive voltage is applied to the base of transistor
86 through resistor 90 causing current to flow through resistor 92
and the base-emitter junction of transistor 86. A second transistor
94, normally biased "off" by resistor 92, is then turned "on" by
the voltage drop across resistor 92, causing current to flow
through the emitter-collector junction of transistor 94 and the
motor 20. A reverse biased diode 96 is connected between the motor
20 and ground to shunt transients to ground which are generated by
the motor 20 in order to protect the transistor 94.
In operation the individual D applying the electrodes applies the
reference electrode 16 and the electrode 10 to the patient as
described above. The individual D then places the drive head 28a of
the preparation device 12 against the cup for the abrasive,
electrolyte-impregnated pad which is rotatably mounted in the
housing of electrode 10. The device 12 is urged against the
electrode 10 with sufficient axial force to compress the coil
spring 36 so that the pressure of the device 12 against the
electrode 10 is relatively constant during the preparation
procedure. The AC signal from the oscillator 60 is applied to the
reference electrode 16, causing a contant current to flow to ground
through the reference electrode 16, patient P, electrode 10 and the
drive member 28. An AC signal having a peak amplitude, which is
proportional to the impedance between the electrodes 16 and 10, is
thus generated at the output of amplifier 66 and rectified by diode
72 to produce a DC voltage indicative of the impedance at the
skin/electrode 10 interface. The individual D then manually
actuates switch 42 to trigger the one-shot 82 and cause the current
amplifier 84 to apply power to the motor 20. Since the motor 20 has
not yet begun to rotate, the rotatably mounted pad, the initial
impedance between the electrodes 10 and 16 is relatively high so
that the voltage applied to the first terminal of the comparator 74
is substantially greater than the reference voltage from the
potentiometer 80. Rotation of the motor 20 rotates the pad in the
electrode 10 to abrade skin beneath the electrode 10, thereby
causing the impedance at the interface between the skin and
electrode 10 to continually decrease. When this impedance drops to
a level corresponding to the reference voltage from potentiometer
80, the one-shot 82 is reset causing the current amplifier 84 to
cut off power to the motor 20. Rotation of the abrasive pad of the
electrode 10 is thus terminated. The preparation device 12 is then
used to prepare the skin beneath another applied electrode. If, for
some reason, the impedance of the skin/electrode interface does not
fall to the value preset by the potentiometer 80 within a
predetermined period, the internal circuit components of the
one-shot 82 cause it to reset after a predetermined duration,
thereby causing the current amplifier 84 to cut off power to the
motor. Thus, abrasion of the skin beneath the electrode 10
automatically terminates before the skin of the patient is
excessively abraded.
If the individual D fails to insert a plug connected to the
reference electrode 16 into jack 50, the lead from the jack 50 is
connected to ground. Consequently, the voltage applied to amplifier
66 is zero, so that the voltage applied to the first terminal of
the comparator 74 is less than the voltage reference from
potentiometer 80. Under these circumstances the one-shot 82 is
reset as soon as it is triggered, so that rotation of the motor 20
is prevented.
The preparation device described enables quick and easy preparation
of the skin beneath the medical electrode 10. The skin preparation
is, in every case, uniform and sufficient to provide proper
electrical contact beteen the electrode 10 and the patient without
excessive skin abrasion.
* * * * *